The suppression of photodarkening is one problem with fiber lasers using optical fibers which have been doped with Ytterbium (Yb).
Photodarkening is a phenomenon in which the laser output gradually decreases over time when light (excitation light) is irradiated onto an Yb-doped optical fiber (see Non-patent documents 1, 2, and 4).
This temporal change is a problem which is specific to Yb-doped optical fibers from among rare earth doped optical fibers. For example, in an Er-doped optical fiber which is used in a fiber amplifier, the problem of a reduction in initial amplification characteristics because of concentration quenching exists. However, photodarkening (i.e., temporal change) which is the subject of the present invention is not found to be a problem. Namely, the reduction in the initial amplification characteristics of the Er-doped optical fiber amplifier is a completely different phenomenon from photodarkening which is the subject of the present invention.
The mechanism behind photodarkening is not clear at the present point. However, there are several examples of research on the matter. Examples of research related to photodarkening in Yb-doped optical fibers are given below.
(Research Example 1)
The creation of color centers in silica glass affects photodarkening. The creation of a color center is permanent damage, and has a center of the absorption wavelength in the visible range. The absorption peak of a color center is in the visible range. However, the absorption band thereof is broad, and the edges of the absorption also have an effect on the infrared region. Accordingly, a loss is inflicted on both excitation light and laser oscillation light by the creation of a color center, which causes the power conversion efficiency of a fiber to decrease (see Non-patent document 3).
(Research Example 2)
If excitation light of the same wavelength is used, then the speed of the deterioration caused by photodarkening increases as the excitation light intensity becomes stronger (see Non-patent document 1).
(Research Example 3)
In the same way as in Research example 1, it is assumed that the creation of color centers is a participating factor. Seven trivalent Yb ions participate in the photoionization in color center creation (see Non-patent document 2).
(Research Example 4)
The cause of photodarkening originates from the fact that the silicon network of the optical fiber glass receives permanent damage. This type of damage is generated by photoionization created by the multiphoton absorption process of excitation light and signal light. An example of a bandgap in silica glass with doped Yb as a rare earth is approximately 5.2 eV (238 nm), and this is smaller than that of undoped silica glass (9 eV). Because a bandgap of 238 nm is approximately four times the energy of the wavelength of the excitation light or signal light (i.e., 1000 nm), there is clearly a possibility that four-photon absorption also plays a part (see Non-patent document 4).
As is the case in Research examples 1 to 4, there are many examples in which it is speculated that color centers (known as defects) in glass which are generated by excitation light or signal light are associated with the mechanism of photodarkening. However, in all the reports, the information is insufficient to enable the cause of photodarkening to be specified, and any clear cause is not yet known. Moreover, it is also not yet known for certain that absorption in the visible regions definitely has an effect on the infrared regions.
In Patent document 1, the method described below is employed in order to improve the resistance of silica glass to ultraviolet rays. As a first step, glass defects are generated by irradiating electromagnetic waves onto a silica object. As a second step, this is immersed into a hydrogen gas atmosphere.    [Patent document 1] Japanese Patent Publication No. 2980094    [Non-patent document 1] “Photodarkening in Ytterbium-Doped Silica Fibers”, Proc. SPIE5990, 72-81 (2005)    [Non-patent document 2] “Population Inversion Factor Dependence of Photodarkening of Yb-Doped Fibers and Its Suppression by High Aluminum Doping”, T. Kitabayashi et al., OFC2006, Anaheim, USA, Paper OThC5, 2006    [Non-patent document 3] “Linear and Non-Linear Photoionization of Silicate Glasses, L. B. Glebov, Glass Science and Technology, Vol. 20, No. 24, 1995”    [Non-patent document 4] “Photodarkening: Understanding and Mitigating”, Liekki White Paper